Method for forming a quickly hardening, inorganic foam

ABSTRACT

A process is provided for forming a rapid-hardening inorganic foam, based on the reaction of the following two components: (a) a solid component in the form of a reactive powder having at the same time structure-building and pore-forming properties, and (b) a liquid component in the form of an alkali metal silicate (water glass). The reactive powder contains the essential constituents: 45-65% by weight aluminum oxide, 10-20% by weight aluminum nitride, and 5-15% by weight metallic aluminum. The alkali metal silicate has a molar ratio of silicon oxide to metal oxide of 1.0 to 2.2. The two components are mixed in a weight ratio of powder to liquid component of 0.5 to 2 to give a paste from which a foamed body having a bulk density of less than 0.7 g/cm 3  is then formed in an exothermal reaction within less than 10 minutes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Section 371 of International Application No. PCT/IB2013/000869, filed Apr. 8, 2013, which was published in the German language on Oct. 24, 2013, under International Publication No. WO 2013/156852 A1 and the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The invention relates to a process for forming a rapid-hardening inorganic foam based on the reaction of two components:

1) a solid component in the form of a reactive powder having at the same time structure-building and pore-forming properties, and

2) a liquid component in the form of an alkaline activation solution.

The invention further relates to a reactive powder for the preparation of a rapid-hardening inorganic foam by reaction with an alkaline activation solution.

Inorganic foam (mineral foam) is used as a thermally insulating, fire-resistant lightweight construction material. Rapid hardening of the foam is of advantage in the filling up of hollows in wall constructions, in the production of insulating boards, and in the repair of the thermal insulation of industrial furnaces. In on-site processing it is advantageous to use a formula that requires mixing of only two components whenever possible. For thermal insulations of industrial furnaces the inorganic foam must be built up of components which offer high temperature resistance.

It is known to produce inorganic foam in the form of a geopolymeric foam by mixing at least three components:

1) reactive solids, containing SiO₂ and Al₂O₃ in sufficient amount and in reactive form (ashes, active clays, meta kaolin, pozzolans, slags, etc.),

2) an alkaline activation solution that contains (apart from water) alkali hydroxides, alkali silicates, alkali aluminates, alkali carbonates and alkali sulphates, either singly or in combinations thereof, and

3) a pore-forming component (addition of foam, gas-forming reaction with aluminum powder or hydrogen peroxide).

See: A. Buchwald, “What are geopolymers? Current state of research and technology, the opportunities they offer, and their significance for the precast industry,” BFT International Vol. 72, No. 7, pp. 42-49 (2006).

U.S. Pat. No. 3,396,112 describes a process of preparing inorganic foams by reacting water with a particulate mixture of aluminum and a dry, water-soluble alkali metal silicate, the particles of the mixture having a particle size of less than 10 mesh (2 mm), and the water content constituting from 30 to 75 weight percent of the total mixture. The weight ratio of the alkali metal silicate to the aluminum is from 15:1 to 1:9, and the alkali metal silicate has an alkali metal oxide to silica ratio of 2:1 to 1:4. According to the description of this patent (column 2, lines 55-58), commercially available aluminum powder or other forms of finely-divided aluminum having the corresponding particle size can be employed, and it is not necessary for the aluminum to be pure. Inert fillers such as silica, alumina, titania, and similar metal oxides can also be added (column 3, lines 8-11).

According to Example 1 of this patent, a foam product was generated by stirring 12.5 g water into a prefabricated dry mixture of 10 g aluminum powder (particle size less than 400 mesh) and 7.5 g solid sodium silicate (molar ratio Na₂O:SiO₂=1:3.22 and containing 17% by weight of water of hydration), while the formation of hydrogen and the connected generation of foam began after 15 seconds, and the mixture solidified after some minutes, connected with a temperature increase to 105° C.

According to Example 2 of this patent, a dry mixture of 30 g of an aluminum dross (with a 70% metal content and a particle size of about 100 mesh) and 70 g of a sodium silicate (particle size about 200 mesh, molar ratio Na₂O:SiO₂=1:2, without water of hydration) is described, to which was added 60 g of water; a foam formed within 30 minutes, which was then dried and hardened for three hours at 110° C.

U.S. Pat. No. 3,784,385 concerns the production of a refractory gas concrete having a thermal resistance up to 1,200° C., consisting of five components:

1) a binder containing sodium silicate,

2) a finely ground filler in the form of a chrome-aluminum slag, in a weight percentage of 22 to 32 percent relative to the total weight when mixed with the binder,

3) addition of water,

4) a finely ground high-alumina refractory material, and

5) a gas-forming agent in the form of sodium hydroxide and aluminum powder.

In accordance with the description to this patent (column 2, line 67 to column 3, line 16), the mixture is heated to 38-42° C. for 3-5 hours, further treated for 10 hours in an autoclave at 170-180° C., and finally stored for another three days at 20° C.

U.S. Patent 4,133,691 describes a process for preparing an inorganic foam, having a molar ratio SiO₂:Al₂O₃=1:1 to 10:1, by mixing particulate metallic aluminum with an aqueous basic solution formed from a water-soluble alkali metal oxide, alkali metal hydroxide or alkali metal aluminate.

European Patent EP 1 180 504 B1 concerns a pore-forming, solidification-accelerating additive for binding agent-building materials, containing 50-90% Al₂O₃, 4-20% MgO, 0.5-15% SiO₂, 0.1-5% aluminum nitride, 0.1-10% metallic aluminum, 0.1-15% loss on ignition, and the main mineral constituents in the form of corundum (α-Al₂O₃) and spinel (MgO×Al₂O₃), wherein the metallic aluminum particles are enveloped by mineral transition modifications from aluminum hydroxide (Al₂O₃×3H₂O) to α-Al₂O₃, and wherein the powder has a particle size of at least 90% smaller than 500 μm and a specific surface of at least 10 m²/g. This additive is produced from a residue from the treatment of aluminum salt slag, by thermal dehydration and activation within a temperature range of 400-1000° C. According to Example 5, test 6, of this patent, 450 g of “SEROX TK750” (powder, thermally activated at 750° C., with a specific surface area of 45 m²/g and a content of metallic aluminum of 3%) was mixed for about 1 minute with 400 g of liquid water glass. After a few minutes, the formation of gas in the paste began, resulting in the formation of foam and an increase in volume, connected with an increase of temperature to 76° C. After a reaction time of 10 minutes the body was hardened so as to be mechanically stable, having a bulk density of 0.63 g/cm³.

European patent application publication EP 0 576 254 A2 concerns a reactive non-metallic product with 7-25% aluminum nitride and 2-8% aluminum, which is produced by treating aluminum dross and aluminum salt slag by washing out the chlorides (page 2, lines 53-55). A possible use of this product for the production of refractories or abrasives is described, but not for the formation of inorganic foam.

From the processing of aluminum dross with metal contents of 40-75% employing a plasma method (see U.S. Pat. No. 4,960,460) the reactive powder “NOVAL” is known, which contains 53-65% aluminum oxide, 9-20% aluminum nitride and 5-12% metallic aluminum. The usability of “NOVAL” for the production of calcium aluminate, sodium aluminate, magnesium aluminate spinel, fused corundum, mineral fibers, as a cement supplement and as a sand-blasting medium is described, but not a possible suitability for the formation of inorganic foam.

See also: R. Breault, S. P. Tremblay, J. Lachance, Y. Huard, “Market Opportunities for the ALCAN Plasma Dross Residues,” Light Metals 823-827(1995); and R. Breault, D. Guay, G. Dubé, D. Legault, R. Morin, K. Annett, J. Bonneau, “Aluminum Plasma Dross Treatment Process and Calcium Aluminate Production: Closing the loop with no residue,” Light Metals 1183-1194 (2000).

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process of forming a rapid-hardening inorganic foam which, based on the reaction by two components, hardens in less than 10 minutes to give a stable body of foam, without requiring an external heat supply to achieve hardening, and, in addition, without the need of drying the foam body. In addition, the mineral foam should have constituents which offer a high fire resistance. A further object of the present invention is the provision of a reactive powder for the formation of a rapid-hardening inorganic foam.

The object is achieved by a process of forming a rapid-hardening inorganic foam based on the reaction of two components:

a) a solid component in the form of a reactive powder having at the same time structure-building and pore-forming properties, and

b) a liquid component in the form of an alkali metal silicate (water glass), characterized in that the reactive powder comprises the essential constituents:

45-65% by weight aluminum oxide,

10-20% by weight aluminum nitride,

5-15% by weight metallic aluminum, and

has a particle size of at least 90% by weight smaller than 1 mm, and wherein the two components are mixed in a weight ratio of powder to liquid component of 0.5 to 2 to give a paste from which a foamed body having a bulk density of less than 0.7 g/cm³ is then formed in an exothermal reaction within less than 10 minutes.

The object is further achieved by a reactive powder for the forming of a rapid-hardening mineral foam by reaction with an alkaline activation solution, wherein the reactive powder comprises the essential constituents:

45-65% by weight aluminum oxide,

10-20% by weight aluminum nitride, and

5-15% by weight metallic aluminum,

and has a particle size of at least 90% smaller than 1 mm.

The object is further achieved by the use of a powder recovered from aluminum dross and which contains the essential constituents:

45-65% by weight aluminum oxide,

10-20% by weight aluminum nitride, and

5-15% by weight metallic aluminum,

as a reactive powder for the forming of an inorganic foam, particularly a lightweight construction material.

The object is further achieved by inorganic foams or lightweight construction materials, which can be obtained by the above process, preferably a thermally insulating and/or fire-resistant lightweight construction material.

Further advantageous embodiments of the invention are described below and in the dependent claims.

DETAILED DESCRIPTION OF THE INVENTION

Since, in general, no heat supply from the outside is required for solidification of the inorganic foam, the process according to the present invention can, preferably, be carried out at room temperature, the room temperature generally being in the range from 15° C. to 30° C., more particularly from 18° C. to 25° C.

In the prior art, it has always been only the effect of metallic aluminum as a gas-forming component which has been considered in connection with the formation of inorganic foam. Surprisingly it has been found that it is possible to form a stable body of mineral foam having a bulk density of less than 0.7 g/cm³, within less than 10 minutes and without the need of further drying, by reaction of a reactive powder having the main constituents 45-65% by weight aluminum oxide, 10 -20% by weight aluminum nitride and 5-15% by weight metallic aluminum, and preferably having a higher content of aluminum nitride than of metallic aluminum, with liquid alkali metal silicate. Furthermore, the high content of aluminum oxide is of advantage for a high fire resistance of the inorganic foam. By x-ray diffraction analysis it can be seen that the mineral composition of the formed bodies of foam still contains some residual unreacted metallic aluminum, while aluminum nitride cannot be detected any more, and that, hence, the aluminum nitride has reacted virtually completely.

These results were unexpected since, in accordance with U.S. Pat. No. 3,396,112, Example 1, when using pure aluminum powder in reaction with a solid water glass and addition of water, the formation and solidification was completed already after a few minutes; however, according to U.S. Pat. No. 3,396,112, Example 2, the formation of the foam already took 30 minutes, when using aluminum dross having a metal content of still 70% by weight.

The rapid forming of gas and solidification of the foam that was found to occur when using the powder according to the invention, being at the same time structure-building and pore-forming, in reaction with a liquid alkali metal silicate, is presumably based on the ammonia gas (from the reaction of AlN+2H₂O==>AlOOH+NH₃) that is released in addition to the hydrogen gas being formed (from the reaction 2Al+4H₂O==>2AlOOH+3H₂), and the connected, additional formation of active aluminum hydroxide gel, which accelerates the formation of the framework structure by polymerization and consolidation of the alkali metal silicate in the way of a geopolymer.

A powder according to the invention, having structure-building as well as gas-forming properties, can be obtained, for example, in the recovery of aluminum from aluminum dross by mechanical crushing and classifying in the form of grain size fractions of less than 1 mm, having the lowest content of aluminum, for example in the form of the screenings “CAI-ALON S-D” and the filter dust “CAI-ALON B-H,” (manufacturer: Cast Aluminum Industries, Dubai, United Arab Emirates; Table 1). As can be seen from Table 1, the aforementioned powders of the present invention exhibit a BET surface area of 2.2 and 3.4 m²/g, respectively.

As aforementioned, as the reactive powder according to the present invention, a powder may be used that has been recovered from aluminum dross, preferably a powder recovered from aluminum dross by mechanical size reduction and classification (particularly by screening, filtration or air classification)

According to the invention other reactive powders are also suitable, if they show the properties according to the invention, for example “NOVAL” obtained according to U.S. Pat. No. 4,960,460 or the non-metallic product according to EP 0 576 254 A2.

The BET surface area (specific surface area) of the reactive powder according to the present invention is preferably smaller than 10 m²/g, more preferably smaller than 7.5 m²/g, more particularly smaller than 5 m²/g. BET surface area is determined according to ISO 9277 (1995), which replaces DIN 66131 (ISO 9277:1995; “Determination of the specific surface area of solids by gas adsorption using the BET method”).

The reactive powder and the alkaline activation solution can of course also be used in combination with further mineral raw materials, such as metakaolin, fly ash, slag, pozzolan, cement, lime, gypsum, sand, glass fibers, mineral fibers, if necessary adding further alkaline activation solution or water in an amount necessary for adjusting the consistency of the mixture to give a pasty consistency.

The present invention further relates to an inorganic foam or lightweight construction material that can be manufactured, or has been manufactured, using the process according to the present invention. More particularly, this foam or material is a thermally insulating or/and fire-resistant lightweight construction material.

The inorganic foam or lightweight construction material according to the present invention may also be present in the form of finished parts made therefrom, particularly insulating boards.

In the subsequent Examples 1-6 (Table 3) the reactive powders “CAI-ALON S-D” and “CAI-ALON B-H” were mixed with an alkaline activation solution, based on the potassium water glass “SILIRIT M 110” (Table 2), in quantitative ratios of powder to water glass from 0.6 to 2 in a plastic mug and stirred to a paste at room temperature for approx. 30 sec, by using a spoon. The reaction temperature was measured by a thermometer introduced into the paste. After the formation and solidification of the foamed body, the bulk density of the body was determined by measuring its weight and outlines.

The results show that the consistency of the paste was readily spreadable with the Examples 1-3, and the formation, the solidification and the bulk density of the foam were best; that is, at mixture ratios of solid to activation solution of a weight ratio of 1:1. The consistency of the paste in Examples 4 and 6 was relatively thin, and in Example 5 relatively thick, so that with regard to achieving formation of an inorganic foam that is as rapid as possible, and with regard to obtaining an inorganic foam having a bulk density that is as low as possible, the best suitable mixing ratios between solid and activation liquid is considered to be in the range of 0.5 to 2. By x-ray diffraction analysis it could be confirmed that the mineral composition of the formed foam bodies still contained some residual metallic aluminum, but no aluminum nitride, and that, hence, the aluminum nitride had reacted virtually completely, in contrast to the metallic aluminum.

TABLE 1 Reactive powders CAI-ALON S-D CAI-ALON B-H Screenings Filter dust Al₂O₃ [%] 49.9 54.9 AlN [%] 12.6 15.0 Al metal [%] 10.9 13.7 SiO₂ [%] 9.6 5.3 MgO [%] 4.9 5.3 CaO [%] 3.3 1.4 Na₂O [%] 0.8 0.4 K₂O [%] 0.6 0.9 Fe₂O₃ [%] 0.4 0.4 TiO₂ [%] 0.5 0.3 F [%] 0.7 2.0 Cl [%] 1.3 0.5 N [%] 4.3 5.1 C [%] 0.7 0.2 Moisture 0.1 0.1 105° C. [%] Change in weight +7.7 +14.5 1100° C. [%] Specific surface 2.2 3.4 area* [m²/g] Particles <300 μm [%] 90 99 Main mineral constituents Corundum +++ +++ Spinel ++ ++ Aluminum Nitride ++ ++ Aluminum Metal ++ ++ Quartz + + *BET surface area; determination using the BET method (ISO 9277)

TABLE 2 Alkaline Water Glass Type: SILIRIT M 110 Supplier: Cognis GmbH, D-40551 Düsseldorf/Germany Density [kg/cm³] (20° C.) 1600-1700 Viscosity [mPas] (20° C.) 40-80 K₂O [%] 27.80-31.00 SiO₂ [%] 22.20-23.00 Weight Ratio [% SiO₂/% K₂O] 0.74-0.80 Molar Ratio [mol SiO₂/mol K₂O] 1.16-1.26

TABLE 3 Examples Example 1 2 3 4 5 6 CAI-ALON S-D [g] 20 — 10 15 25 10 CAI-ALON B-H [g] — 20 10 — — 10 SILIRIT M110 [g] 20 20 20 25 15 10 Water [g] — — — — — 10 Mixing ratio 1 1 1 0.6 1.66 2 solid:water glass Consistency of the a) a) a) b) c) b) paste Time at start of 1 1 1 1 1 4 reaction [min] Temperature 126 132 126 130 110 93 max. [° C.] Solidification [min] 5 5 5 6 3 7 Bulk Density [g/cm³] 0.37 0.38 0.37 0.43 0.67 0.57 a) readily spreadable b) relatively thin c) relatively thick

It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims. 

1-22. (canceled)
 23. A process of forming a rapid-hardening inorganic foam based on reaction of two components (a) and (b) as follows: a) a solid component in a form of a reactive powder having structure-building and pore-forming properties, and b) a liquid component in a form of an alkali metal silicate, wherein the reactive powder comprises as essential constituents: 45-65% by weight aluminum oxide, 10-20% by weight aluminum nitride, 5-15% by weight metallic aluminum, and has a particle size of at least 90% by weight smaller than 1 mm, the process comprising mixing the two components in a weight ratio of the powder to the liquid component of 0.5 to 2 to yield a paste, and forming a foamed body having a bulk density of less than 0.7 g/cm³ in an exothermal reaction within less than 10 minutes.
 24. The process in accordance with claim 23, wherein the two components are mixed for a maximum of five minutes, optionally a maximum of one minute.
 25. The process in accordance with claim 23, wherein the two components are mixed without supply of heat from the outside, optionally at room temperature, to yield the paste.
 26. The process in accordance with claim 23, wherein the alkali metal silicate is in a form of a solution having a molar ratio of silicon oxide to metal oxide of 1.0 to 2.2.
 27. The process in accordance with claim 23, wherein the reactive powder has a weight ratio of aluminum nitride to metallic aluminum of greater than
 1. 28. The process in accordance with claim 23, wherein the reactive powder has a particle size of at least 90% by weight smaller than 300 μm.
 29. The process in accordance with claim 23, wherein the reactive powder is mixed with a liquid potassium water glass, having a molar ratio (SiO₂:K₂O) of 1.16 to 1.26.
 30. The process in accordance with claim 23, wherein the reactive powder and the alkali metal silicate are used in combination with at least one additional mineral raw materials selected from metakaolin, fly ash, slag, pozzolan, cement, lime, gypsum, sand, glass fibers, and mineral fibers.
 31. The process in accordance with claim 30, wherein, if necessary, further alkali metal silicate or water is added in a quantity necessary for adjusting a pasty consistency of the mixture.
 32. The process in accordance with claim 23, wherein a BET surface area of the reactive powder is smaller than 10 m²/g, optionally smaller than 7.5 m²/g or smaller than 5 m²/g.
 33. The process in accordance with claim 23, wherein the reactive powder is a powder which has been recovered from aluminum dross, optionally a powder recovered from aluminum dross by mechanical size reduction and classification.
 34. A reactive powder for forming a rapid-hardening mineral foam by reaction with an alkali metal silicate, wherein the reactive powder comprises as essential constituents: 45-65% by weight aluminum oxide, 10-20% by weight aluminum nitride, and 5-15% by weight metallic aluminum, and has a particle size of at least 90% smaller than 1 mm.
 35. The reactive powder in accordance with claim 34, wherein the reactive powder has a weight ratio of aluminum nitride to metallic aluminum of greater than
 1. 36. The reactive powder in accordance with claim 34, wherein the reactive powder has a particle size at least 90% by weight smaller than 300 μm.
 37. The reactive powder in accordance with claim 34, wherein the reactive powder has a BET surface area smaller than 10 m²/g, optionally smaller than 7.5 m²/g or smaller than 5 m²/g.
 38. The reactive powder in accordance with claim 34, wherein the reactive powder has been recovered from aluminum dross, optionally by mechanical size reduction and classification.
 39. A method for forming an inorganic foam, particularly a lightweight construction material, by reaction of a solid component in a form of a reactive powder having structure-building and pore-forming properties, wherein the reactive powder has been recovered from aluminum dross and contains as essential constituents: 45-65% by weight aluminum oxide, 10-20% by weight aluminum nitride, and 5-15% by weight metallic aluminum.
 40. The method according to claim 39, wherein at least 90% by weight of the reactive powder has a particle size of less than 1 mm, optionally less than 300 μm.
 41. The method according to claim 39, wherein the reactive powder is reacted with a liquid component in the form of an alkali metal silicate to form the inorganic foam.
 42. The method according to claim 39, wherein the reactive powder has a weight ratio of aluminum nitride to metallic aluminum of greater than
 1. 43. The method according to claim 39, wherein the reactive powder has a BET surface area smaller than 10 m²/g, optionally smaller than 7.5 m²/g or smaller than 5 m²/g.
 44. An inorganic foam or lightweight construction material formed by the process according to claim 23, wherein the foam or construction material is at least one of a thermally insulating and a fire-resistant lightweight construction material.
 45. The inorganic foam or lightweight construction material according to claim 44, wherein the foam or construction material is present in a form of a finished part, optionally in a form of an insulating board. 